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November 22, 2004

Whitworth Cannon

I got a request in an earlier thread for pictures of the Whitworth rifled breech loader breech and bolt. Bolt in this instance referring to the round it shot (or at least I hope that's what the requester was after!).

I've got some stuff in the reference library - but I didn't have any good pictures of the breech mechanism to scan, so I went hunting on the web. And, as I expected, about all I could find was this, the most common photo of a Whitworth, from the Civil War. I found some other British guns, but none of those shots showed the breech to any good effect.

But joy of joys, after a couple of refinements in my Googling, I came up with these photos. They are from this website, devoted to the hobby of making and shooting miniature cannon. This may be the avenue the Arsenal has to go in order to indulge our taste in cannon.

Anyway, here are two pretty good shots of the Whitworth - in model form, made by a remarkable mini-cannon-founder, Ronald Nulph.

The Whitworth was a "screw-gun," meaning that it's breech block worked exactly like a screw - requiring multiple twists of the breech handle to close and seal the breech. Developed at a time before brass cartridges cases of that size were practical, they were plagued by sealing problems at the breech over time, in addition to some of the inherent weaknesses in the wrought-iron construction methods used.

These problems would so plague the screw-guns that first rank armies of the era went back to rifled muzzle-loaders until a solution was found in the form of the 'interrupted screw' breech and the french-designed DeBange obturation system. The interrupted screw breech (still preferred on large guns) with the DeBange sealing system allows for the breech to close and seal in a quarter-turn, vastly speeding service of the piece. The DeBange obturator was essentially a mushroom-shaped steel spindle that sat in the center of the breech block. It sat on a split ring, obturating pad (usu. a hard, heat resistant rubber or asbestos compound) with another split ring on top of it. The compression of firing pushed the mushroom back on the split rings and obturator, which bulged to seal the breech. The charge is initiated by a primer (looks like a large blank) inserted into the lock. Just like a rifle cartridge case, the brass case seals the lock, the pad seals the breech, the interrupted screw allows a quarter turn to seal, giving you a very strong, very fast breech for large caliber guns. The various forms of dropping and sliding blocks (as used on smaller guns and tank guns) give even greater speed - but at the cost of weight, which is why larger caliber guns use stepped thread screw breeches - with at least the exception of the German 155mm guns, which still use blocks. The stepped screw breech still soldiers on, however - as this picture of Redleg Marines sending a present via their M198 Howitzer to muji's in Fallujah amply demonstrates.

The diagram above is a DeBange interrupted screw breech in a naval gun. The cannoneers on the Marine gun would recognize the essentials of this breech.

The second part of the question was the Whitworth bolt. Bolt, in artillery parlance of the Civil War era, meant an elongated rifled projectile that did not explode - the rifled equivalent of solid shot (in this case, a 30pdr Parrot bolt).

The reason a Whitworth bolt is interesting is because the Whitworth gun (designed, incidentally by Sir Joseph Whitworth) used a novel method of rifling. Rather than cutting grooves into the bore of the piece to spin the projectile, the Whitworth gun's bore was hexagonal in section, and twisted down the bore to provide the spin to stabilize the projectile, and provide a predictable drift that could be offset in aiming.

Consequently, the ammunition had to be specially made to accommodate that - which gives you a projectile that looks like this.

Seen behind the bolt is a 12pdr spherical case (exploding shell) with a Bormann fuze.

Obviously, one of the last things the Confederates needed was a gun that required specialized ammunition. So, while the Whitworth was an accurate gun, it's propensity in it's wrought iron mode to explode without warning, and the requirements for specially-made ammunition, combined with it's relative lack of power made it a not terribly useful gun. But what Whitworth learned in the design of this gun and his rifles was carried forward part and parcel into the guns we cannon-cockers use today.

There, that should about cover it. I really could go on for pages, but this is a blog, eh?